1. Field of the Invention
The present invention relates to a pressure control device for a pressure roller, and more particularly to a pressure control device for controlling the application of pressure provided by a pressure roller such as a transfer roller and a pressing roller for a fixing device to a pressing object in an image forming apparatus including the same.
2. Description of the Prior Art
An image forming apparatus such as a printer and a copying machine is provided with a pressure roller for applying a pressure to a pressing object such as a photosensitive drum. Such a pressure is conventionally controlled by the methods as described below.
FIG. 10 shows one of the conventional pressure control devices for a pressure roller. The device comprises a lever 50 which pivotally moves around a horizontal support 51 and supports a transfer roller 5 at one end thereof. The transfer roller 5 is disposed below a photosensitive drum 1. The transfer roller 5 normally presses the photosensitive drum 1, and the pressure is released by manually pushing the opposite end of the lever 50 to the transfer roller 5, against the urging force of a spring 52. This operation is required, for example, when jamming occurs between the photosensitive drum 1 and the transfer roller 5.
Since the above conventional device is manually operated, good operability is not possible, and a longer time is required to release the jamming. Further, the transfer roller 5 normally presses the photosensitive drum 1 with the urging force of the spring 52, which over time may cause deformation of both the photosensitive drums 1 and the transfer roller 5.
FIG. 11 shows a second type of the conventional pressure control device a pressure roller, in which a solenoid 6 is added to the above device together with a plunger 60 for coupling the solenoid 6 to the lever 50. When jamming occurs, the solenoid 6 is turned on so as to allow the plunger 60 to withdraw. This is followed by the pivotal movement of the lever 50 in such a direction that the transfer roller 5 supported thereto moves away from the photosensitive drum 1 and the pressure therebetween is thereby released.
In the above conventional device, the lever 50 is directly moved by the solenoid 6. This structure is disadvantageous in that it requires a large solenoid and, as a result, the image forming apparatus becomes complicate and large, requiring further costs.
FIG. 12 shows a third type of the conventional pressure control device, in which the lever 50 is pivotally moved by the combination of the solenoid 6 and a spring clutch 7 so as to release the pressure of the transfer roller 5 from the photosensitive drum 1. Normally, the solenoid 6 is not energized, and a sleeve 70 of the spring clutch 7 is engaged with a ratch 61 coupled to the plunger 60 of the solenoid 6, so as to lock a cam 71 of the spring clutch 7 in order to prevent it from rotating. When jamming occurs and the solenoid 6 is turned on, the cam 71 is unlocked and starts to rotate, allowing the lever 50 to pivotally move in such a direction that the transfer roller 5 supported thereto moves away from the photosensitive drum 1 and the pressure therebetween is released.
The above conventional device still has a disadvantage. When the main electric source of the image forming apparatus is turned off, the main motor which drives various drive systems thereof stops. In this respect, when the main electric source is turned off with the transfer roller 5 pressing the photosensitive drum 1, the pressure application is maintained. Thus, if the image forming apparatus is kept unused for a long period of time, both the photosensitive drum 1 and the transfer roller 5 may be deformed due to the pressure against each other.
The image forming apparatus employing the above conventional pressure control devices is normally provided with a timing roller which transports a sheet so that it passes between the photosensitive drum 1 and the transfer roller 5. The timing roller is conventionally provided with a solenoid other than the above solenoid 6 so as to control the rotation of the timing roller. As a result, the structure of the apparatus becomes large and complicated with the increased number of components, and this also makes it difficult to realize cost reduction.
An objective of the present invention is to solve the above-described prior art deficiencies by providing a pressure control device for a pressure roller which ensures the prompt release of jamming and which prevents both a pressing object and a pressure roller from being deformed when the image forming apparatus is left unused for a long period of time.
Another objective of the present invention is to simplify the structure of the image forming apparatus and to achieve cost reductions thereof.